Biaxially oriented polyester films (for example, polyethylene terephthalate films, which are commonly referred to as PET films) have widely been used in the fields of insulating materials, drafting and photographic films, magnetic tapes, vacuum metalized films and various packaging materials because they are excellent in electrical insulating properties, transparency, dimensional stability and toughness.
However, these biaxially oriented polyester films are insufficient in adhesion of the surface because molecular chains of the resin constituting the films are highly oriented and are therefore inferior in adhesion to a printing ink, magnetic coating composition, deposited metal or adhesive which is applied on the films.
In order to improve adhesion to the printing ink by activating the surface of the polyester film, the surface of the films is commonly subjected to a corona treatment, ultraviolet irradiation treatment, plasma treatment or flame treatment. Although adhesion can be improved by these treatments, the activity deteriorates with time. Therefore, means for activation of the surface of these films is not necessarily satisfactory.
As the other method of improving adhesion of the film surface, there is suggested an etching method of swelling or dissolving the surface using chemicals such as acid, alkali, trichloroacetic acid and phenols. This method enables decomposition and dissolution of crystal orientation in the vicinity of the film surface and also deteriorates cohesiveness thereby to improve adhesion, thus ensuring the effect. However, some of these chemicals are harmful and involve some risk upon handling.
As a method similar to this method, there is suggested a method of previously forming a thin coating layer (which is referred to as a primer layer, anchor layer or undercoat layer) made of the material, which is different from that of a base material, on a film using a coating agent which is referred to as a primer coating agent or anchor coating agent.
There has been used a method of applying a solvent-based coating agent prepared by dissolving in an organic solvent on a surface layer of a film in the prior art, however, an aqueous coating agent (which is referred to as an aqueous primer coating agent or aqueous anchor coating agent) has commonly been used, recently, for reasons of safety and sanitation. Particularly, an aqueous polyurethane resin composition having excellent adhesion to various base materials has been studied as the aqueous coating agent.
Plastic films comprising a coating layer made of the aqueous coating agent are used for various purposes. Among these plastic films, a polyester film is widely used as a food packaging film because of its excellent transparency and toughness. In these purposes, polyester film is not usually used by itself and there is commonly used a laminated film obtained by printing on a plastic film and laminating the plastic film with various films using an adhesive. Therefore, processability during printing or lamination under various conditions as well as excellent durability are required for the plastic film comprising the coating layer formed thereon. Since various compositions such as solvent-based resin composition and aqueous resin composition are used as the ink and adhesive to be coated on the coating layer formed on the plastic film, excellent solvent resistance, water resistance and heat resistance are required for the coating layer formed on the plastic film. To respond to these demands, the aqueous coating agent is commonly used in combination with various crosslinking agents.
Recent diversification in the film has required various functions for the film. Particularly, plastic films comprising a coating layer made of polyvinylidene chloride or a deposit layer made of aluminum formed for the purpose of improving gas barrier performances of a packaging material are used as a composition of the packaging material. In that case, a method of previously applying a coating agent on the surface of a plastic to form a coating layer is used for the purpose of improving adhesion to the layer made of polyvinylidene chloride or aluminum deposit. To impart excellent gas barrier performances, such a coating layer must be uniformly formed. This requires that the coating agent can be applied to various coating methods and can always form a uniform coating layer even under various conditions.
As the method of applying a coating agent on the plastic film, there are employed an in-line coating method of applying a coating agent during the biaxial stretching step in the manufacturing process of a plastic film and performing the lateral stretching step, and an off-line coating method of applying a coating agent to the taken-up plastic film performing the biaxial stretching step, and drying the coating agent to form a coating layer.
In the in-line coating method, since the coating agent is applied before crystal orientation is completed during the biaxial stretching step of the plastic film, adhesion between the base film and the coating layer is improved. In case of a PET film, the stretching step is further performed at the temperature of 150° C. or higher after applying the coating agent. In that case, reaction with the coating agent and crosslinking agent is so fast, and the coating layer cannot conform to the base material during the stretching step, and thus cracking occurs in the coating layer.
In the off-line coating method, since dimensional stability of the base material must be maintained, the drying step is commonly performed at a low temperature of 100° C. or lower. Therefore, a solvent-based coating agent capable of forming a hard and uniform coating layer at low temperature was used. However, it is not preferable in view of safety and problems such as environmental pollution and thus it is required to develop an aqueous coating agent which can also be applied to the off-line coating method.
Various suggestions have been made as a measure to counter these problems.
For example, there is disclosed that an aqueous resin composition comprising an aqueous polyester-urethane resin having a pendant carboxyl group neutralized with ammonia or organic amine in a high-molecular weight compound obtained from a polyester polyol having an aromatic cyclic structure and a polyisocyanate compound can provide a primer coating agent which is excellent in water resistance and blocking resistance (see, for example, Patent Document 1).
According to this method, a hard aqueous polyurethane resin can be obtained by using a polyester having an aromatic cyclic structure. Because of high glass transition temperature of a polyurethane resin, the film forming process at high temperature is required in order to obtain a uniform coating layer.
Actually, various crosslinking agents are often used in combination for the purpose of improving solvent resistance and heat resistance of the coating layer. In that case, there arises a problem that an uniform coating layer is hardly formed because of poor conformability to the base material during the stretching step of the film in the in-line coating method.
For the purpose of solving such a problem and forming a uniform coating layer, there is employed a method of plasticizing a resin as a main component in a coating agent by using an organic solvent having a high boiling point as a film forming agent.
In the method of using the film forming agent, for example, there is suggested a polyurethane emulsion coating agent composition wherein MFT (Minimum Film Forming Temperature) is lowered and film forming properties at normal temperature or lower temperature are improved by using an oxyalkylene glycol derivative having a boiling point of 80 to 270° C., which is a hydrophilic fusing agent, as the film forming agent (see, for example, Patent Document 2).
There is suggested a vacuum metalized polypropylene film wherein adhesion to a base material is improved by using a water-soluble organic solvent such as N-methyl-2-pyrrolidone as a film forming agent and mixing it with a polyester-urethane resin (see, for example, Patent Document 3).
However, these methods using the organic solvent have problems, for example, harmfulness to the human body and poor safety such as inflammation, and therefore it is required to develop a coating agent which contains no organic solvent and is excellent in film forming properties at lower temperature. Also in case of modifying the surface of plastic films having poor adhesion such as polypropylene, nylon and polyester films, it has been required to develop a coating agent which is excellent in adhesion between the coating agent and the base material.
When using the organic solvent as the film forming agent, there arises a problem that the film forming agent remains in the coating film and thus surface hardness decreases and wear resistance and blocking resistance deteriorate in case drying conditions at comparatively low temperature of 100° C. or lower are required, like the off-line coating method, and also there arises a problem that the water resistance deteriorates if the coating film is not sufficiently dried because the film forming agent has high hydrophilicity.
As the method of improving adhesion while maintaining heat resistance, there is suggested an aqueous polyurethane dispersion which is obtained by adding a polymer for improving adhesion to a polyurethane resin and dispersing the mixture in water (see, for example, Patent Document 4).
In this aqueous dispersion, a trial of improving adhesion and heat resistance is made by adding a resin having a softening temperature of 80 to 130° C., as a polymer for improving adhesion, to a polyurethane resin. When the aqueous dispersion by means of such a method is applied to the in-line coating method, conformability to the base material during the stretching step deteriorates, and thus it is difficult to obtain a uniform film free from coating film defects. When applied to the off-line coating method at lower processing temperature, the aqueous dispersion has poor film forming properties, and thus it is difficult to obtain a uniform film free from coating film defects.    (Patent Document 1) Japanese Patent Application, Second Publication No. Hei 8-22900 (page 1, left column to page 2, right column)    (Patent Document 2) Japanese Patent Application, Second Publication No. Sho 63-14748 (page 1, left column to page 2, right column)    (Patent Document 3) Japanese Patent Application, First Publication No. 2000-108262 (page 2, left column, claim 1 to page 2, right column, paragraph “0005”)    (Patent Document 4) Japanese Patent Application, First Publication No. Hei 5-230364 (page 1, left column to page 2, right column)